Abstract : High-rate quenching experiments have established that iron containing less than 0.0017%C transforms martensitically at 750C at cooling rates in excess of 35 000 degC/s. M(s) falls to 540C, and the critical quenching speed is lowered to 5 000 degC/s when the carbon content is increased to a figure between 0.005 and 0.01%. Two hypotheses are offered to rationalize the effect of carbon, and Crussard's shock-wave model of the martensitic transformation is extended. (Author)